SH1 Mangamuka Gorge slip repairs

Project update

16 April 2025

Slip A7 – Crews working on the final major task of installing the waler (support beam).

SH1 Mangamuka Gorge slip repairs – April 2025 update

Kia ora koutou,

There has been a lot of activity since our last update. We’ve completed anchor installations at Slips A7 and A9, boxed and filled all the anchor pockets, finished the motorcycle rail works, and tidied up completed sites by removing construction debris and leftover materials.

We’re pleased to report that we have now completed installation of all anchors in the gorge. The last major construction step – installing the waler beam at Slip A7 – is underway. This horizontal beam is essential for reinforcing and supporting the vertical piles, helping to distribute the load and adding stability to the structure.

After the Easter and ANZAC holiday weekends, we’ll be back for a short period to complete the testing of anchors at A7, finish the final site cleanup and remove the last of the equipment.

Temporary traffic management will remain in place while we complete the final stages of work. However, all traffic management will be lifted with both lanes open over the Easter and ANZAC holiday weekends.

Slip A11 update

Following a recent storm that brought 170mm of rain on 4 April, our team observed new movement and minor fall out on Slip A11. This was promptly cleaned up, with no disruption to road users. Another section above this area has also shown signs of movement.

This part of the slope was intentionally cut during earlier works, with geotechnical oversight, and some gradual movement over time was expected. With the challenging geology of the gorge, small overslips like these are expected from time to time and are generally much easier to manage than the more complex underslips we’ve repaired throughout the project.

While the risk of major slips through the gorge has been significantly reduced, the area remains susceptible to occasional minor movement due to the ground conditions in the Maungataniwha Range. However, because of the stabilisation work we’ve completed, any future movement is expected to be much less severe, helping us respond quickly and minimise disruptions.

The slip is still within the buffer zone created during last year’s excavation, and we’re continuing to monitor the area closely. Our crews are on standby and ready to respond quickly if needed.

Slip A11 - Material that came loose during the heavy rain on 4 April.

Slip A11 - After cleanup. Our team is closely monitoring this area.

Thank you for your continued patience while we complete this work. Please continue to take care when traveling through the gorge and around the worksites to ensure the safety of both workers and road users as we wrap up these final tasks.

Ngā mihi nui,

The SH1 Mangamuka Gorge slip repairs project team

Aerial view of SH1 through Mangamuka Gorge.

More information

For more information on the SH1 Mangamuka Gorge slip repairs, contact us at northlandproject@nzta.govt.nz

Visit our website www.nzta.govt.nz/mangamuka